In such a turbomachine, the purpose of adjusting the angular position or “pitch” of certain stator vanes is to optimize the efficiency of the turbomachine and to reduce its fuel consumption under differing flight configurations. Such adjustment is generally performed for one or more rows of vanes by means of a control ring which surrounds the outside of the turbomachine stator and which can be turned about the longitudinal axis of the stator by drive means such as an actuator or an electric motor. Turning movement of the ring is transmitted by links to the vanes of the row, each link being secured at one of its ends to a vane and carrying a radial finger at its other end, which finger is engaged in a cylindrical housing in the control ring.
In turbomachines presenting a certain power level, the casing of the stator is made up of two semicylindrical shells including longitudinal junction flanges which project outwards from the casing. Because of the presence of such longitudinally-extending flanges, each control ring is made up of two approximately semicircular elements rigidly interconnected at their ends by two bridges that are placed astride the flanges of the casing and that allow the ring to turn through a certain angle about the longitudinal axis of the casing.
The radial fingers of the links extend inwards when they are designed to be received in housings of the substantially semicircular ring elements, and they extend radially outwards when they are designed to be received in cylindrical housings of the bridges, since the bridges are further away from the longitudinal axis of the casing that are the semicircular elements of the ring.
As a result, the points of rotation of the links connected to the bridges are further away from the longitudinal axis of the casing than are the points of rotation of the links connected to the semicircular elements of the ring, and angular displacement of the ring about the axis of the casing leads to the vanes connected by links to the semicircular ring elements being turned through a certain angle while the vanes connected by links to the bridges of the ring are turned through a greater angle.
This different adjustment of the vanes connected to the bridges impedes optimization of efficiency and fuel saving.